Timing calibration for fast signal reacquisition in navigational receivers

ABSTRACT

The present invention provides GPS receivers with clock calibration for fast reacquisition of GPS signals after waking up from a sleep state or coming out of signal blockage. In a preferred embodiment, a GPS receiver comprises a local clock based on an oscillator, e.g., crystal oscillator. The GPS receiver calculates a clock calibration value based on a computed oscillator count for the period during which the GPS receiver is in the sleep state or the signal is blocked. This clock calibration value is used to calibrate the local clock after the GPS receiver wakes up or comes out of signal blockage for fast reacquisition of GPS signals.

FIELD OF THE INVENTION

The present invention relates to satellite based navigational receivers,and more particularly to techniques for fast reacquisition ofnavigational signals after a navigation receiver experiences a signalblock or wakes up from a sleep mode.

BACKGROUND

The global positioning system (GPS) is a satellite basedradio-navigation system built and operated by the United StatesDepartment of Defense. The Russian government operated ‘GLONASS’ andEuropean Union proposed ‘Galileo’ are two other important satellitebased navigational systems.

GPS permits a user of the system to determine his or her position on thesurface of the earth. The system consists of twenty-four satellitescircling the earth at an altitude of about 11,000 miles with a period ofabout 12 hours. It is possible to have more than twenty-four satellitesdue to the presence of some spare satellites in the GPS constellation.These satellites are placed in six different orbits such that at anytime a minimum of six and a maximum of more than eleven satellites arevisible to any user on the surface of the earth except in the polarregion. Each satellite transmits an accurate time and position signalreferenced to an atomic clock. A typical GPS receiver locks onto thissignal and extracts the data contained in it and with signals from asufficient number of satellites, a GPS receiver can calculate itsposition, velocity, altitude, and time.

Sometimes GPS receivers are required to operate under very weak signalconditions as in foliage or indoors. In the present day practice, thereceiver may get “assistance” in the form of additional acquisitionaiding messages from a server or base station, or Internet based. Butproviding this type of assistance requires additional infrastructure andmay not be available in all places. Also, the receiver requiresadditional hardware to receive the aiding messages. Therefore there is aneed to develop GPS receivers that operate in “standalone” mode underweak or indoor signal conditions. Further, there is a need as in thecase of E911 (Enhanced 911), for fast acquisition of the GPS signals. Inaddition to the above, the power saving in the receiver is also animportant requirement.

Most of the standalone high sensitivity GPS receivers are based on along non-coherent integration involving squaring loss and thus reducingthe possible gain while taking a long time to acquire the satellitesignal under weak signal conditions. Regarding the prior art in thisfield, published U.S. Patent Application 2003/0164795 deals with a typeof sleep mode where the clock is kept alive during power off. However,this disclosure does not take into account variations in oscillatortemperature and signal Doppler. U.S. Pat. No. 5,893,044 uses an accuratereal time clock when the signal is interrupted due to blockage. U.S.Pat. No. 6,320,536 discloses a compensated clock using WAAS (Wide AreaAugment System) signals. U.S. Pat. No. 6,662,107 discloses a powersaving mode which uses a separate clock circuit that is powered all thetime. This clock circuit also provides corrections for temperaturevariations. The clock correction claimed in that patent (claim 19) isbased upon the temperature and Doppler. U.S. Pat. No. 6,725,157describes a procedure wherein the GPS receiver first operates outdoorand then maintains the lock when moved indoor. U.S. Pat. No. 6,757,610discloses storing some parameters like clock Doppler, receiver velocityfor use during the next acquisition or reacquisition.

Present day GPS receivers do not make extensive use of clockcalibration. Therefore, there is a need for GPS receivers that use clockcalibration to make faster reacquisition of GPS signals possible.

SUMMARY OF THE INVENTION

The present invention provides GPS receivers with clock calibration forfast reacquisition of GPS signals after waking up from a sleep state orcoming out of signal blockage.

In a preferred embodiment, a GPS receiver comprises a local clock basedon an oscillator, e.g., crystal oscillator. The GPS receiver calculatesa clock calibration value based on a computed oscillator count for theperiod during which the GPS receiver is in the sleep state or the signalis blocked. This clock calibration value is used to calibrate the localclock after the GPS receiver wakes up or comes out of signal blockagefor fast reacquisition of GPS signals.

In another embodiment, the oscillator is a temperature compensatedcrystal oscillator TCXO and the computed oscillator count comprises aTCXO count.

In yet another embodiment, the clock calibration value is compensatedfor relative motion between the GPS receiver and a GPS satellite duringthe sleep or signal blockage period.

In yet another embodiment, the clock calibration value is compensatedfor frequency drift of the oscillator due to changes in temperature.

In yet another embodiment, the GPS receiver compensates the codefrequency of the locally-generated pseudo random sequence for Dopplershift when the GPS receiver wakes up or comes out of signal blockage.

In yet another embodiment, the GPS receiver compensates the phase codeof the locally-generated pseudo random sequence for changes in distancebetween the GPS receiver and GPS satellite during the sleep or signalblockage period.

The above and other advantages of embodiments of this invention will beapparent from the following more detailed description when taken inconjunction with the accompanying drawings. It is intended that theabove advantages can be achieved separately by different aspects of theinvention and that additional advantages of this invention will involvevarious combinations of the above independent advantages such thatsynergistic benefits may be obtained from combined techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a is a functional block diagram of a GPS receiver according toan embodiment of the invention.

FIG. 2 is a functional block diagram of a Delay Lock Loop (DLL)accumulation process according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a functional block diagram of a GPS receiver according to anembodiment of the present invention. An RF front-end 100 processes theRF signal received at the antenna (not shown). Operations of aconversional RF front-end 100 include amplification, down-conversion,and analog-to digital conversion. The RF front end 100 outputs anintermediate frequency (IF) signal 101 to a baseband section of thereceiver from its analog-to-digital converter (ADC) output (not shown).The RF front-end 100 down converts the received RF into the intermediatefrequency (IF) for baseband processing. The IF signal 101 is madeavailable to two paths, one in-phase (I) and the other in-quadrature(Q). In the I path, the IF signal 101 is multiplied in IF mixer 102in-phase with a local frequency signal generated by a direct digitalfrequency synthesizer (DDFS) 106 to produce the in-phase (I) component107. In the Q path, the same signal 101 is multiplied by the DDFSfrequency in-quadrature (i.e., with a phase shift of 90 degrees) toproduce the quadrature (Q) component 108. The DDFS 106 is driven by acarrier numerically controlled oscillator (NCO) 105. In addition,carrier NCO 105 receives phase and frequency corrections from aprocessor 113. Because of this correction, the DDFS frequency and phaseis almost the same as that of IF signal 101 As a result, the I and Qsignals produced by the IF mixers 102 and 103 are near zero carrierfrequency. In other words, the outputs I 107 and Q 108 of the IF mixers102 and 103 are stripped or wiped off from the carrier (IF). The I and Qsignals may be low-pass filtered to remove the high frequency componentswhich are equal to twice the IF frequency band.

The I and Q components 107 and 108 are correlated in correlators 109 and110, respectively, with a locally-generated pseudorandom (PN) sequencegenerated by a PN generator 111. The PN-sequence corresponds to thechannel being processed by the baseband section at that time. The PNsequence generator is driven by code NCO 112. The local code generatorfrequency is made equal to the code rate of the I and Q paths bycorrective feedback from the processor 113 to the code NCO 112. Inaddition, the processor 113 sends a signal to PN code generator 111 toset the starting phase of the locally generated code. The NCO 112provides the correct clock signals to correlators 109 and 110. Forexample, NCO 112 provides a clock signal to generate two samples per PNchip in the signal acquisition stage and three samples per chip duringthe tracking stage. SYS CLK 104 provides to NCO 105 and NCO 112 a commonclock synchronization signal. The correlator output values are then sentto processor 113 at millisecond intervals in the form of 1 ms samples,where each 1 ms samples is the result of correlation of one PN sequenceof length 1023 chips. The processor 113 may be a digital signalprocessor (DSP) core. Preferably, the processor is capable of performingfast math intensive operations. Subsequent processing of the signalstakes place in the processor 113, as will be described in detail below.Additional details of the receiver baseband section described above canbe found in copending U.S. patent application Ser. No. 11/123,861,titled “Efficient And Flexible GPS baseband Architecture,” filed on May6, 2005, the specification of which is incorporated in its entiretyherein by reference.

The processor 113 receives one millisecond integrated (correlated) I andQ values from the GPS baseband section described above. In order toacquire a GPS signal in the processor 113, all dwells (i.e., pairs ofcarrier frequency and code phase values) are searched. This is atwo-dimensional search. Coherent integration and non-coherentintegration are two commonly used integration methods to acquire GPSsignals. Coherent integration provides better signal to gain at the costof greater computation load for equal integration times.

The GPS receiver also includes a Temperature Compensated CrystalOscillator (TCXO), which provides a local reference frequency for systemtiming, the local receiver clock, down-conversion to the IF signal,sampling timing, and other clock related actions in the GPS receiver.Crystal oscillators are commonly used to provide reference frequenciesfor GPS receivers. The TCXO is a crystal oscillator that is temperaturecompensated to minimize frequency variations due to changes intemperature. The TCXO is included in the SYST CLK 104 as shown inFIG. 1. The GPS receiver also includes a Delay Lock Loop (DLL) forsynchronizing the locally-generated PN sequence with the PN sequence ofthe GPS signal by providing the PN code generator 111 with accurate codefrequency and code phase.

FIG. 2 illustrates an DLL correction accumulation process duringtracking. The I and Q component signals 107 (I) and 108 (Q) of the FIG.1 are inputted to the DDL. The I and Q correlators 109 and 110 of FIG. 1are the prompt correlators. The I and Q correlators 154 and 155 are theEarly (E) correlators while I and Q correlators 156 and 157 are the late(L) correlators. The reference local PN sequence is advanced by 0.5 chipin the case of the E correlators 154 and 155 as shown in block 158.Similarly, the local PN sequence is delayed by 0.5 chip in the case ofthe L correlators 156 and 157 as shown in block 159. The DLL correctionwhich is obtained as the difference of the outputs of the E and Lcorrelators is accumulated in memory block 163. This correction signalalso changes the code NCO 112 in such a way that the E and L correlatorswill have same outputs and the prompt correlators 109 and 110 will havethe maximum output in 165, thus indicating a perfect synchronization.

In some situations, the GPS receiver may be unable to continuouslyreceive GPS signals. The break in signal reception may be due to signalblockage as experienced under foliage. The GPS receiver may also be in apower saving mode, in which the GPS receiver switches between a sleepstate and a wakeup state to save power. Examples of GPS receivers withpower saving modes can be found in copending U.S. patent applicationSer. No. ______, titled “Unassisted Indoor GPS Receivers,” filed on thesame date as the present application, the specification of which isincorporated in its entirety by reference. In these cases, the GPSsignal has to be reacquired often. To help in this reacquisition, anestimation of the acquisition parameters is made using the collectedTCXO count and Delay Lock Loop (DLL) compensation. The TCXO count andDLL compensation are collected when the receiver is in reception of asufficient number of signals from satellites and is able to compute itsposition.

The time period during which the GPS receiver can remain without asignal due to signal blockage or sleep state depends on how well timinginformation is maintained. The better the timing information accuracy,the longer the signal blockage or sleep state time may be. Further, thereacquisition of GPS signals may be faster with accurate parameters. Tomaintain good timing information, a time matching and compensationscheme is implemented according to an embodiment of the invention. Inthis embodiment, a 40-bit counter is used to count TCXO pulses. Thiscounter counts TCXO during the power on period and is incremented duringthe start of the wakeup period as explained below. During a period of xseconds (e.g., 18 seconds) of power on or no signal blockage, the GPSsatellite is tracked and the TCXO count for a duration of x*50 bits ofGPS navigation data (e.g., 18 seconds corresponds to 900 bits) is thecounter value. During the same period, the DLL compensation values fromthe input to the NCO 112 of the PN code generator 111, and temperaturev/s time curve are also obtained and stored in memory. The variousparameters needed for the fast reacquisition of the GPS signal arecomputed as below.

TCXO Compensation

This section discusses TCXO compensation for calibrating the localreceiver clock after the GPS receiver has awaken from a sleep state orhas come out of signal blockage. The receiver computes a TCXO count forthe period of the sleep state. Since the time of the sleep state orpower off is known, the correct frequency at wakeup can be calculated asTCXO count/(sleep state period). If the count is more over the sleepstate period, then the frequency is assumed increased. The correctfrequency is used to calibrate the local receiver clock. Thiscalibration decreases the frequency search range upon wake up. If notcalibrated, the receiver has to search a larger frequency range as itdoes not know how much the frequency has changed during the sleepperiod. The receiver searches the frequencies by dividing the possiblefrequency range into several bins and searching for the signal in eachbin. The more bins that need to be searched, the more time it takes toreacquire the satellite signal. If frequency variation is compensated,then the number of frequency bins to search is less and so the satellitesignal may be acquired in a shorter time.

Computation of TCXO Count During Power Off or Signal Blockage

In this embodiment, the receiver switches to a sleep state or signalsare blocked after the receiver has tracked a GPS satellite for aduration of x*50 navigation data bits, where x is in seconds and thedata bits are transmitted at a rate of 50 bits/second. The TCXO countimmediately prior to switching to the sleep state or no signal conditionis TCXO1. TCXO1 is the TCXO count during the duration of x*50 navigationdata bits. In this embodiment, the GPS receiver powers on after yseconds in the sleep state or after a duration of y*50 navigation databits. For example, y seconds may be the time of the next super-frame,which corresponds to a duration of 1500×24 bits.

Let TXCO2 be a scaled value of TCXO count over y seconds (e.g., to thestart of the next super-frame). TXCO2 may be given by:TCXO2=TCXO1*(y*50)/(x*50)

Thus, TCXO2 is computed by scaling TCXO1 by the ratio of (y*50)/(x*50).For the example of tracking the GPS satellite for 900 bits (i.e., 18seconds) and sleeping until the next super-frame (1500×24 bits), theratio is (1500×24 bits)/(900 bits).

Thus, TCXO2 provides an initial estimate of the duration of signalblockage or sleep time in terms of TCXO count.

Relative Motion Compensation

The next step is to correct the TXCO count for the relative motionbetween the GPS satellite and the GPS receiver during the sleep time orblockage time. In this step, the distance between the satellite andreceiver are computed at the time the satellite goes into sleep mode(receives last bit before going into sleep mode). This distance isdenoted D1. D1 may be a pseudorange measurement made during satellitetracking before the receiver goes to the sleep state. The receiverperiodically updates the pseudorange while tracking usually once persecond. The distance is also calculated between the satellite andreceiver at the time the receiver wakes up from sleep mode or isunblocked. This distance is denoted D2. To calculate D2, the position ofthe satellite at wakeup can be calculated using ephemeris data oralmanac data previously received by the GPS receiver and stored inmemory before going to sleep. As is well known, ephemeris data providesorbital data parameters for the GPS satellite and other parameters forcalculating the position of the GPS satellite as a function of time.Almanac data provides orbital data parameters for all the GPS satellitesand can be used to approximate the position of any GPS satellite. Bothalmanac data and ephemeris are available for all the satellites. Thereceiver position at wakeup may be approximated by the stored receiverposition taken before the sleep state. For example, if the receiver isindoors (e.g., office building) during the sleep state, then it may beassumed that there is less movement of the receiver. The receiverposition may be also computed by other means such as an odometer and/orGyroscope, which may also be combined to obtain the receiver velocity.

The correction to the TXCO count due to relative motion or Doppler maybe denoted by C1 and may be given by:C1=TCXO frequency*(Dp2−D1)/(speed of light)

where the TCXO frequency is the frequency of the TCXO pulses. The TCXOfrequency may be 16 MHz. This frequency may be multiplied or divided asrequired in the receiver by synthesizers. The speed of light may beassumed to be constant at 300,000 Kms/second.

Temperature Drift Compensation

This step is not required for the TXCO because the TCXO has circuitrythat compensates for temperature variations for a more constantfrequency. This is because TCXOs are temperature compensated and so thefrequency variation may be assumed to be small over a comparativelyshort sleep period of, e.g., 12.5 minutes. However, this step may benecessary for an uncompensated crystal oscillator (XO) to compensate theXO count for frequency drift due to temperature changes. This is becauseXOs are not temperature compensated and the frequency variation may belarge. In this case, a temperature versus frequency drift table may beused to correct the XO count for frequency drift due to temperaturechanges. This table may be stored in memory on the receiver. In caseswhere temperature changes, and hence the frequency drift, are small,this step may be ignored.

Let the temperature when the receiver goes to sleep be Ti and thetemperature when the receiver is powered on be Tf. The temperature maybe detected by a temperature sensor located near the crystal oscillator.Then the temperature difference is Td=Tf−Ti. The frequency changecorresponding to Td from the table=fd. Let the change in the TXCO countdue to fd=C2.

For example, the temperature may be measured every 6 seconds and the fdmay be assumed constant for this 6 seconds. The corresponding count forthis interval is fd*(6 seconds). C2 may be obtained by summing the countof each interval during the sleep period. For a sleep period of 732seconds, there will be 122 intervals.

In cases where a frequency drift table is not available, a rate ofchange of frequency drift with temperature may be pre-computed andstored in memory. This rate of change has to be computed for differentpoints of temperature versus frequency drift curve.

Also, in cases where a frequency drift table is not available, such atable may be generated during the power on period of the receiver bypartitioning the power on period into small time intervals andtabulating temperature versus frequency drift at each of theseintervals. The generated table may be used for the temperature rangenormally experienced by the receiver.

Final Corrected TCXO

The final corrected TXCO count for the period of sleep or signalblockage may be given by:Final corrected TXCO=TCXO2+C1+C2

For a temperature compensated crystal oscillator, C2 is not required.

This final corrected value is used to calibrate the local clock at thetime the GPS receiver powers on or comes out of signal blockage. Becauseof this accurate estimation of the clock through the final correctedTCXO, a fast reacquisition of the GPS signal is possible. In the case ofa known sleep state period and no temperature compensation, the TCXOcount may be computed before going to sleep. Otherwise the TCXO count iscalculated later.

Delay Lock Loop Compensation

To synchronize the locally-generated PN sequence with the PN sequence ofthe GPS satellite when the receiver wakes up or comes out of blockage,code frequency and code phase corrections are needed. Such correctionsmay be needed for every satellite because each satellite has a differentPN sequence.

To compute code frequency and code phase, some prior information suchthe receiver time, receiver position, GPS time, satellite position,effective navigation information (e.g., ephemeris, almanac, ect.) areneeded. The value of position fixed and all the above information arestored in memory before the receiver goes to sleep or signal blockage.

Code Frequency Compensation for Doppler Shift due to Velocity ofSatellite

The first step is to compensate the code frequency at the instance thereceiver wakes up for the Doppler shift due to the velocity of thesatellite.

If the sleeping duration is accurate enough, the pseudorange between thesatellite and the receiver at the instance the receiver wakes up fromthe sleep state can be calculated. The pseudorange is a first orderapproximation of distance and can be calculated using the storedephemeris data. To calculate the pseudorange, the position of thesatellite can be calculated from stored ephemeris. The receiver positionmay be approximated as the previously stored receiver position assumingthere is little movement in, e.g., an indoor environment. Alternatively,the receiver position may be calculated using an odometer and/orGryoscope. The relative velocity (rate of change of pseudorange) of thesatellite to receiver may be obtained by calculating the pseudorange attwo different times and dividing by the time difference. This gives thepseudorange rate variation along the direction of the receiver andsatellite. Based on the velocity, the Doppler shift of the codefrequency due to satellite velocity at that moment can be predicted. Theregister of Code_Frequency can be updated when the receiver wakes up.

Phase Code

The second step is to compensate the phase code for the change inpseudorange or distance from the receiver to the satellite between thetime that the receiver goes to sleep or signal is blocked and the timethat the GPS wakes up from the sleep state or comes out of signalblockage. The pseudoranges may be calculated as explained above. Basedon the pseudorange difference and the navigation information (e.g.,ionsphere parameters), the code phase difference can be calculated. Forexample, the code difference can be calculated by dividing thepseudorange difference by the length of one code period. The length ofone code period is 1 ms*speed of light with no compensation for Dopplershift. The code length can be readily calculated taking Doppler shiftinto account, where the Doppler shift is calculated based on therelative velocity of the satellite to receiver. The code length iscompressed due to Doppler shift when the satellite is moving toward thereceiver and expanded due to Doppler shift when the satellite is movingtoward the receiver.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the “true” spirit and scope of theinvention.

1. A method for calibrating a local clock of a navigation receiver,wherein the local clock is based on an oscillator, comprising:calculating a clock calibration value based on a computed oscillatorcount for a period during which the receiver is in a sleep state orunder signal blockage; and calibrating the local clock using the clockcalibration value during reacquisition of GPS signals when the receiverwakes up from the sleep state or comes out of signal blockage.
 2. Themethod of claim 1, wherein the computed oscillator count is given by:OX2=OX1*y/x where OX1 is an oscillator count during a period of x, and yis the period of the sleep state or signal blockage, and OX2 is thecomputed oscillator count.
 3. The method of claim 1, wherein theoscillator is a temperature compensated oscillator (TCXO) and theoscillator count is a TCXO count.
 4. The method of claim 1, wherein theclock calibration value is compensated for Doppler frequency changesduring the period of the sleep state or signal blockage.
 5. The methodof claim 1, wherein the clock calibration value is compensated forfrequency drift of the oscillator due to temperature changes during theperiod of the sleep state or signal blockage.
 6. The method of claim 1,wherein the clock calibration value is compensated for relative motionbetween the receiver and a navigational satellite during the period ofthe sleep state or signal blockage.
 7. The method of claim 5, whereinthe temperature compensation is computed from a frequency drift versustemperature table.
 8. The method of claim 2, wherein the clockcalibration value is further compensated for relative motion between thereceiver and a navigational satellite during the period of the sleepstate or signal blockage.
 9. The method of claim 8, wherein the clockcalibration value is further compensated for temperature changes duringthe period of the sleep state or signal blockage.
 10. The method ofclaim 1, further comprising compensating a code frequency of alocally-generated signal for Doppler shift due to a velocity of anavigational satellite when the receiver wakes up or comes out of signalblockage.
 11. The method of claim 10, wherein the locally-generatedsignal is a pseudorandom sequence signal.
 12. The method of claim 1,further comprising compensating a code phase of a locally-generatedsignal for a change in distance from the receiver to a satellite betweena time the receiver goes to the sleep state and a time the receiverwakes up from the sleep state.
 13. The method of claim 1, wherein thenavigational receiver is a GPS receiver.
 14. The method of claim 1,wherein the navigational receiver is a GLONASS or a Galileo receiver.15. A navigational receiver, comprising: a local clock based on anoscillator; a radio frequency front-end for receiving satellite signals;a baseband section for processing the received satellite signals intocorrelated values; and a processor for integrating the correlated valuesto acquire the satellite signals, wherein the processor calculates aclock calibration value based on a computed oscillator count for aperiod during which the receiver is in a sleep state or under signalblockage, and the processor calibrates the local clock using the clockcalibration value during reacquisition of GPS signals when the receiverwakes up from the sleep state or comes out of signal blockage.
 16. Thenavigational receiver of claim 15, wherein the computed oscillator countis given by:OX2=OX1*y/x where OX1 is an oscillator count during a period of x, and yis the period of the sleep state or signal blockage, and OX2 is thecomputed oscillator count.
 17. The navigational receiver of claim 15,wherein the oscillator is a temperature compensated oscillator (TCXO)and the oscillator count is a TCXO count.
 18. The navigational receiverof claim 15, wherein the processor compensates the clock calibrationvalue for Doppler frequency changes during the period of the sleep stateor signal blockage.
 19. The navigational receiver of claim 15, whereinthe processor compensates the clock calibration value for frequencydrift of the oscillator due to temperature changes during the period ofthe sleep state or signal blockage.
 20. The navigational receiver ofclaim 15, wherein the processor compensates the clock calibration valuefor relative motion between the receiver and a navigational satelliteduring the period of the sleep state or signal blockage.
 21. Thenavigational receiver of claim 19, wherein the processor computes thetemperature compensation from a frequency drift versus temperaturetable.
 22. The navigational receiver of claim 16, wherein the processorcompensates the clock calibration value for relative motion between thereceiver and a navigational satellite during the period of the sleepstate or signal blockage.
 23. The navigational receiver of claim 22,wherein the processor compensates the clock calibration value fortemperature changes during the period of the sleep state or signalblockage.
 24. The navigational receiver of claim 15, further comprising:a local pseudorandom sequence generator; and a Delay Lock Loop (DLL) forsynchronizing the pseudorandom sequence generator with a pseudorandomsequence of a satellite signal, wherein the processor computes a codefrequency compensation for the DLL to compensate for Doppler shift dueto a velocity of a navigational satellite when the receiver wakes up orcomes out of signal blockage.
 25. The navigational receiver of claim 15,further comprising: a local pseudorandom sequence generator; and a DelayLock Loop (DLL) for synchronizing the pseudorandom sequence generatorwith a pseudorandom sequence of a satellite signal, wherein theprocessor computes a code phase compensation for the DLL to compensatefor a change in distance from the receiver to a satellite between a timethe receiver goes to the sleep state and a time the receiver wakes upfrom the sleep state.
 26. The navigational receiver of claim 15, whereinthe navigational receiver is a GPS receiver.
 27. The navigationalreceiver of claim 15, wherein the navigational receiver is a GLONASS ora Galileo receiver.